The present invention relates generally to telecommunication techniques. More particularly, the invention provides an encoding and decoding system and method that support a plurality of compression standards and share computational resources. Merely by way of example, the invention has been applied to Code Excited Linear Prediction (CELP) techniques, but it would be recognized that the invention has a much broader range of applicability.
Code Excited Linear Prediction (CELP) speech coding techniques are widely used in mobile telephony, voice trunking and routing, and Voice-over-IP (VoIP). Such coders/decoders (codecs) model voice signals as a source filter model. The source/excitation signal is generated via adaptive and fixed codebooks, and the filter is modeled by a short-term linear predictive coder (LPC). The encoded speech is then represented by a set of parameters which specify the filter coefficients and the type of excitation.
Industry standards codecs using CELP techniques include Global System for Mobile (GSM) Communications Enhanced Full Rate (EFR) codec, Adaptive Multi-Rate Narrowband (AMR-NB) codec, Adaptive Multi-Rate Wideband (AMR-WB), G.723.1, G.729, Enhanced Variable Rate Codec (EVRC), Selectable Mode Vocoder (SMV), QCELP, and MPEG-4. These standard codecs apply substantially the same generic algorithms in extracting CELP parameters with modifications to frame and subframe sizes, filtering procedures, interpolation resolutions, code-book structures and code-book search intervals.
For example, the GSM standards AMR-NB and AMR-WB usually operate with a 20 ms frame size divided into 4 subframes of 5 ms. One difference between the wideband and narrowband coder is the sampling rate, which is 8 kHz for AMR-NB and 16 kHz downsampled to 12.8 kHz for analysis for AMR-WB. The linear prediction (LP) techniques used in both AMR-NB and AMR-WB are substantially identical, but AMR-WB performs adaptive tilt filtering, linear prediction (LP) analysis to 16th order over an extended bandwidth of 6.4 kHz, conversion of LP coefficients to/from Immittance Spectral Pairs (ISP), and quantization of the ISPs using split-multi-stage vector quantization (SMSVQ). The pitch search routines and computation of the target signal are similar. Both codecs follow an ACELP fixed codebook structure using a depth-first tree search to reduce computations. The adaptive and fixed codebook gains are quantized in both codecs using joint vector quantization (VQ) with 4th order moving average (MA) prediction. AMR-WB also contains additional functions to deal with the higher frequency band up to 7 kHz.
In another example, the Code Division Multiple Access (CDMA) standards SMV and EVRC share certain math functions at the basic operations level. At the algorithm level, the noise suppression and rate selection routines of EVRC are substantially identical to SMV modules. The LP analysis follows substantially the same algorithm in both codecs and both modify the target signal to match an interpolated delay contour. At Rate ⅛, both codecs produce a pseudo-random noise excitation to represent the signal. SMV incorporates the full range of post-processing operations including tilt compensation, formant postfilter, long term postfilter, gain normalization, and highpass filtering, whereas EVRC uses a subset of these operations.
As discussed above, a large number of industry standards codecs use CELP techniques. These codecs are usually supported by mobile and telephony handsets in order to interoperate with emerging and legacy network infrastructure. With the deployment of media rich handsets and the increasing complexity of user applications on these handsets, the large number of codecs is putting increasing pressure on handset resources in terms of program memory and DSP resources.
Hence it is desirable to improve codec techniques.